Several types of pipe joints have been developed over the years for the purpose of solving the leakage problem. Perhaps the most successful type, depicted in U.S. Pat. No. 4,085,951, a special premium cost connection that requires very special care and handling. Said connector has been shown to leak when slight damage occurs on surfaces as at 12 or 14, hence the need for adding the plastic ring 24. However, no provision has been made for thermal expansion of the "compressible ring" and upon an increase in temperature, differential thermal expansion of the plastic ring causes an undesireable increase in the connection hoop stress.
U.S. Pat. No. 2,907,589 explains the susceptability of the Hydril connector to damage, in columns 1 and 2, which may leak due to a variety of reasons. The same patent discloses another plastic seal ring and although "end clearance may be provided to allow for dirt and sand or for machining tolerances" no definite room is provided for thermal expansion of the plastic ring. Such a condition may provide an erratic tendency to seal, giving a false assurance in a dangerous situation.
U.S. Pat. No. 2,980,451 discloses a plastic ring deposed intermediate the engaged threads and confined so as to "extrude" the plastic, clearly allowing for no thermal expansion of the plastic and allowing for a progressive relaxation of the plastic seal which in the presence of changing temperatures and pressures, in time allows leakage. U.S. Pat. No. 3,047,316 also allows for extrusion of a plastic ring but makes no provision for sealing of the plastic ring after thermal bulk contraction occurs.
U.S. Pat. Nos. 3,572,777 and 3,100,656 show soft seals trapped by the end of the male connector but make no provision to protect the soft seals against: blowout by external pressure; erosion and wear from internal abrasive fluid flow.
Conventional pipe threads utilizing tapered threads wherein the pitch diameter of the mating threads lie along only one conical path, see FIG. 1, are still by far the most commonly used, and necessarily have machining tolerance for that taper. After extensive study of experimental and field results over the years, the American Petroleum Institute has recognized that should the male thread have a faster taper than the female thread, even though within tolerance, see FIG. 2, that the seal diameter will be toward the largest diameter of engaged threads which in turn, produces a greater hoop stress on the coupling than would occur had the male thread been machined with a slower taper than the female thread. Therefore, such threaded connections will be derated pressurewise, due to the lesser thread contact pressures that may be generated between the male and female threads; using a given wall thickness of the coupling.
A paper presented by Thomas L. Blose on July 21, 1970 to modify API 8 Round Thread Casing Couplings proposed the use of a second taper within the coupling, toward the small end of the engaged threads, having a steeper taper than the remaining threads so as to ensure the maximum sealing pressure being at the minimum possible seal diameter, thus preserving a higher pressure rating for the connection. However, no such coupling has been made available to the industry to applicants best knowledge. API paper 83-PET presented by the applicants discloses information relative to the instant invention as does their publication in the "Oil and Gas Journal" on Apr. 4, 1983.